1. Field of the Invention
The present invention generally relates to a CDMA (Code Division Multiple Access) communication apparatus and a CDMA communication method. More specifically, the present invention is directed to such a CDMA communication apparatus and a CDMA communication method, which suitably use a multi-code.
2. Description of the Related Art
Conventionally, communication systems in which the CDMA system is used are known in the telecommunication field, such as disclosed in U.S. Pat. No. 4,901,307. In this CDMA communication system, data that is transmitted for every user is spread by employing different spreading codes from each other, and then the spread data is transmitted to each user. Since mutual correlations among these different spreading codes are made small, the data transmitted from the respective users can be discriminated from each other when the spreading codes are inverse-spread.
In accordance with such a CDMA communication system, since the data of a plurality of users can be transmitted/received substantially at the same time by using the same frequency band, a large number of users can be stored within a limited frequency band.
Also, in such a CDMA communication system, if spreading codes whose mutual correlations are equal to zero are employed, then it is possible to eliminate an adverse influence given to own transmission data caused by transmission data of other user.
However, it is practically difficult to select such spreading codes whose mutual correlations are zero. Thus, the transmission data sent from other user may cause noise when the transmission data from his own station is to be received by the counter station. To suppress such noise levels to minimum levels on the reception side, the transmission power of the transmission data is required to be adjusted on the transmission side in such a manner that the reception power of all of the reception data becomes equal to each other.
For instance, a base station in a mobile communication system performs a transmission power control, namely instructs the respective mobile stations to adjust transmission power thereof in such a manner that reception power of reception signals derived from a plurality of mobile stations under management becomes substantially equal to each other. This transmission power control is carried out based on the reception power of the data received by the base station. This sort of transmission power controlling method for mobile stations is referred to as a “closed loop control”.
In the mobile station, not only the transmission power control is carried out in response to such an instruction issued from the base station, but also the transmission power control of the mobile station is carried out based on the reception power of the data received by this mobile station. This sort of transmission power controlling method for mobile stations is called as an “open loop control”. The transmission power control method for the mobile stations by way of both the open loop control and the close loop control is disclosed in U.S. Pat. No. 5,056,109.
On the other hand, very recently, CDMA communication systems in which the multi-code is used are developed. In accordance with this CDMA communication system, while a plurality of spreading codes are used in a single mobile station, a plurality of communications can be substantially simultaneously carried out in this single mobile station. That is to say, in the case that a data communication is wanted to be performed while executing the normal voice (speech) communication, since the voice data and the transmission data are spread by using different spreading codes from each other, both the voice data and the transmission data can be substantially simultaneously transmitted without interference that occurred between the voice data and the transmission data. Also, one piece of relative transmission data is subdivided into a plurality of transmission data, and then the subdivided transmission data are spread by employing the different spreading codes from each other, and further the subdivided/spread transmission data are transmitted substantially at the same time. As a result, the transmission speed of the transmission data can be increased.
FIG. 1 is a schematic block diagram for showing a conventional transmitter in which the multi-code is used.
Referring to FIG. 1, this transmitter is arranged by including a spreading unit 1a for spreading first transmission data by a first spreading code which is internally produced; an FIR (Finite Impulse Response) filter 2a for limiting a frequency band of an output signal from the spreading unit 1a to a preselected range; another spreading unit 1b for spreading second transmission data by using a second spreading code which is internally produced; another FIR filter 2b for limiting a frequency band of an output signal from the spreading unit 1b to a predetermined range; an adder for adding an output signal of the FIR filter 2a to an output signal of the FIR filter 2b; a D/A converting unit 4 for converting a digital output signal of the adder 3 into an analog signal; an analog transmission unit 5 for transmitting the analog output signal of the D/A converting unit 4 in the wireless manner; and an antenna 6 for a wireless signal transmission.
Since the first spreading code used in the spreading unit 1a and the second spreading code used in the spreading unit 1b have the small mutual correlations, two different sorts of transmission data, namely the first transmission data and the second transmission data are spread by the respective spreading codes, and then can be substantially simultaneously transmitted.
On the other hand, actual transmission data is mainly constituted of a pilot signal and a data signal. This pilot signal corresponds to a predetermined signal used to establish the synchronization and the like. The data signal is that actually wanted to be transmitted by a user. The pilot signal that constitutes a portion of the transmission data is employed in not only the first transmission data shown in FIG. 1, but also the second transmission data shown in FIG. 1.
As a consequence, considering simple processing operation such as demodulating process on the reception side, both the pilot signal portion contained in the first transmission data and the pilot signal portion contained in the second transmission data are preferably spread by using the same spreading codes for the transmission purpose. As apparent from the foregoing descriptions, the data signal portion of the first transmission data and the data signal portion of the second transmission data are spread by using the different spreading codes for the transmission purpose.
Now, both the first transmission data and the second transmission data, which have been processed by the above-explained spreading process, are added to each other by the adder 3 shown in FIG. 1. In this case, since the pilot signal portion of the first transmission data and the pilot signal portion of the second transmission data are present in-phase condition, the transmission power thereof is accordingly increased. To the contrary, as to the data signal portion, since both the first transmission data and the second transmission data are spread by using the different spreading codes having the small mutual correlations from each other, the transmission power thereof remains in the original transmission power. As a result, the transmission power of the signal transmitted from the analog transmission unit 5 shown in FIG. 1 is increased, namely the transmission power of the pilot signal portion is different from that of the data signal portion. As a result, the below-mentioned problems (1) and (2) will occur.
(1) When a power difference is produced between the transmission power of the pilot signal portion and the transmission power of the data signal portion, the continuously varied power is reached to the reception circuit on the receiver side. Assuming that the adverse influence caused by the transmission path is ignored, as to the electromagnetic wave reached from the mobile station to the base station in the conventional CDMA communication system without using the multi-code, there is no power difference between the pilot signal portion and the data signal portion, namely constant power in an ideal case.
As previously explained, such a CDMA communication system is required to precisely control transmission power so as to transmit signals with proper transmission power to the respective users. To perform this transmission power control, since the pilot signal portion is used, when there is a power difference between this pilot signal portion and this data signal portion, no proper transmission power control can be performed. In other words, even when Eb/N0 corresponding to the convergence value of the transmission power control can be satisfied while transmitting the pilot signal portion, Eb/N0 cannot be satisfied while transmitting the data signal portion. As a result, when the multi-code is used, in such a case that the transmission power of the data signal portion becomes smaller than the transmission power of the pilot signal portion, the desirable reception quality characteristic cannot be obtained on the reception side. Roughly speaking, there is a proportional relationship between the reception quality characteristic and Eb/N0.
(2) While the internal interpolation synchronous detection with employment of the pilot signal is carried out, there is such a problem that the input/output phase difference is produced. This is caused by the input power difference produced in the amplifying stage for the intermediate frequency band and in the amplifying stage for the high frequency band.
Normally, since the transmission data are continued, while the data signal of the present transmission data is sandwiched between the pilot signal of the present transmission data and the pilot signal of the next transmission data, the resultant data is transmitted. Such a phase rotation of the data signal portion sandwiched by the pilot signals in the transmission path must be corrected. As previously explained, when there is such a transmission power difference between the pilot signal portion and the data signal portion, the rotation elements by cause other than the transmission path are involved. Therefore, the processing operation becomes quite complicate.
Now, a detailed description is made of the reason why such a power difference occurs between the pilot signal portion and the data signal portion.
In a CDMA system, a spreading code is selected in view of the following description:    1) many sorts of signals;    2) small mutual correlations capable of discriminating signals from each other;    3) sharp self-correlation characteristics required for establishing synchronization; and    4) a long time period and appearance at random because of secrecies.
In the case that a plurality of communications are realized at the substantially same time by a single mobile station, a plurality of the above-explained spreading codes may be assigned. In this case, a plurality of spreading codes may be assigned to each the data signal portion of a plurality of transmission data on the communication. The phase-shifted codes may be applied instead of a plurality of different spreading codes from each other. To the contrary, one typical spreading code among the plurality of spreading codes may commonly be assigned to the pilot signal portion of a plurality of transmission data, because of the quick processing operation and of simplicity.
In the above-described assignment case, assuming now that “N” pieces of transmission data stream is expressed as Dn(t)=(1, −1), a total power value of the data signal portion is expressed by the following formula (1):{D1(t)+D2(t)+ . . . +Dn(t)}2=D1(t)2+D2(t)2+ . . . +DN(t)2+2×(D1(t)×D2(t)+D1(t)×D3(t)+ . . . +Dn−1(t)×Dn(t))  formula (1),wherein the “N” and “n” are an integer and “t” is a parameter of time.
Also, the following formula (2) can be satisfied due to the orthogonal characteristic owned by this spreading code:∫D1(t)×D2(t) dt=0  formula (2),where the integral range of this formula (2) corresponds to the time period of the spreading code. When the above formula (1) is modified by using the relationship defined by the formula (2), the below-mentioned formula (3) is obtained:{D1(t)+D2(t)+ . . . +Dn(t)}2=D1(t)2+D2(t)2+ . . . +Dn(t)2=N  formula (3)
In other words, the total power value of the data signal portion may become “N” as expressed in the above formula (3), when the “N” pieces of transmission data stream is simultaneously transmitted by multi-code transmission spread manner.
On the other hand, a total power value of the pilot signal portion may become 32=9. That is, since all of the same spreading codes are employed so as to spread the pilot signal portions, in such a multi-code spreading operation that 3 sets of transmission data are transmitted at substantially same time, when the power values are simply added to each other, the pilot signal owns the amplitude value of either “+3” or “−3”. In other words, the total power value of the pilot signal portions becomes N2 in the case that N pieces of transmission data are transmitted.
As a result, when the total number N of the transmission data is larger than 1, namely when the transmission data is transmitted by the multi-code spreading manner, such a transmission power difference is produced between the pilot signal portion and the data signal portion.